Normally, in a vehicle such as an electric vehicle (EV) or a hybrid vehicle (HV), the drive force based on electric energy is obtained by converting direct-current (DC) power supplied from a battery of high voltage to three-phase alternating-current (AC) power by an inverter, and rotating a three-phase AC motor with the three-phase AC power. In deceleration of the vehicle, conversely the regenerative energy obtained by regeneration of the three-phase AC motor is stored in the battery. Thus, the vehicle runs, making full use of the energy.
In such a hybrid or electric vehicle, the power generation of the generator driven by the engine is controlled such that the state of charge (SOC) indicating the charged quantity of a battery is maintained in a prescribed range. A secondary battery such as a nickel metal hydride battery, lithium ion battery and the like is employed as the battery. Since such a secondary battery is involved in the driving of the vehicle, its high reliability is required.
On the other hand, it is known that the performance of a secondary battery significantly varies depending on its temperature. Among others, the power that can be input to or output from the battery may be greatly decisive for the running performance of the vehicle. The power that can be input to or output from the battery is significantly reduced as the battery temperature is reduced. Accordingly, there has been a problem that, in cold climates or the like, a desired output cannot be obtained in starting the vehicle or in assisting the engine output due to the low battery temperature, and the vehicle cannot be smoothly started or accelerated.
One strategy for the problem may be to use a heater apparatus to increase the battery temperature when the battery temperature is lower than a prescribed temperature. On the other hand, the strategy leaves many problems unsolved. That is, additional components such as the heater apparatus increase size of the vehicle and manufacturing costs. In terms of the energy efficiency, a part of heat generated by the heater apparatus escapes to parts other than the battery.
Therefore, recently, many battery control apparatuses have been disclosed, which efficiently increase the battery temperature to suppress reduction in the power that can be input to or output from the battery, in order to ensure the running performance of the vehicle (for example, see Japanese Patent Laying-Open No. 2003-272712, Japanese Patent Laying-Open No. 2003-274565, Japanese Patent Laying-Open No. 2000-092614, and Japanese Patent Laying-Open No. 2004-015866).
For example, Japanese Patent Laying-Open No. 2003-272712 discloses a battery control apparatus characterized by repeated charging and discharging of a battery in a prescribed range of the state of charge (SOC) of the battery, when the temperature of the battery is equal to or lower than a prescribed value.
The battery control apparatus disclosed therein includes: battery temperature detecting means for detecting a temperature of a battery; state-of-charge determining means for determining a state of charge (SOC) of the battery; and battery control means for controlling charge-discharge of the battery. When the temperature of the battery is equal to or lower than a prescribed value, the battery control means repeatedly and alternately charges and discharges the battery in short cycles in a prescribed range of SOC. Here, the battery control means causes an internal resistance of the battery to generate heat by a current passing through the battery (the battery current) during charge or discharge so that the battery temperature is directly increased from the inside. Accordingly, the temperature can effectively be increased without a power loss, as compared to the case where the battery temperature is increased using a heater apparatus.
Further, the battery control apparatus of Japanese Patent Laying-Open No. 2003-272712 as incorporated in a hybrid vehicle controls charging and discharging of a battery in a temperature increase mode, in accordance with the running state of the vehicle.
Specifically, in the running state of the vehicle, the battery control means executes the battery discharge control by prohibiting driving of a generator for compensating for the battery consumption associated with running of the vehicle (the generation for driving) and regenerative braking of a motor for driving, in a prescribed range of SOC, so that charging of the battery is stopped. It executes the battery charge control by permitting the generation for driving and the regenerative braking so that charging of the battery is started. That is, while the vehicle is running, the battery means executes battery charge and discharge control by prohibiting/permitting driving of the generator by the engine and the regenerative braking of the motor for driving.
In the stop state of the vehicle, the battery control means executes the battery charge control by starting the engine to forcibly drive the generator.
As described above, the battery control apparatus of Japanese Patent Laying-Open No. 2003-272712 prohibits/permits driving of the generator and the motor for driving, or forcibly drives the generator, depending on the state of the vehicle, to thereby control the charging and discharging of the battery to increase the temperature thereof. This invites such a problem that the cycles in which the battery is actually charged or discharged is dependent on the running state of the vehicle, and therefore the battery temperature can hardly be quickly increased. In particular, in a lithium ion battery, the temperature is not easily increased since the internal resistance at low temperatures is lower than the internal resistance at the normal temperature. Therefore, an improvement in the efficiency of increasing the temperature has been required.
Further, prohibition/permission of the generation for driving and the regenerative braking is executed by the battery control unit outputting an instruction for stopping the generation/an instruction for starting the generation to an auxiliary control unit, and by the battery control unit transferring a signal for suppressing torque during the regeneration to a motor control unit via a torque control unit. This complicates the battery charge/discharge control in the temperature increase mode.
Accordingly, the present invention has been made to solve the problems described above, and an object thereof is to provide a secondary battery control apparatus and a secondary battery control method capable of quickly and easily increasing the temperature of the secondary battery.